Display apparatus and electronic apparatus

ABSTRACT

Disclosed herein is a display apparatus, including: a plurality of subpixels disposed adjacent each other and forming one pixel which forms a unit for formation of a color image; the plurality of subpixels including a first subpixel which emits light of the shortest wavelength and a second subpixel disposed adjacent the first subpixel; the second subpixel having a light blocking member disposed between the second subpixel and the first subpixel and having a width greater than a channel length or a channel width of a transistor which forms the second subpixel.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a display apparatus and an electronicapparatus, and more particularly to a display apparatus of the flat typeor flat panel type wherein a plurality of pixels each including anelectro-optical element are arranged two-dimensionally in rows andcolumns, that is, in a matrix, and an electronic apparatus whichincorporates the display apparatus.

2. Description of the Related Art

In recent years, in the field of display apparatus which display animage, a flat type display apparatus wherein a plurality of pixels orpixel circuits each including a light emitting element are arranged inrows and columns, has been popularized rapidly. One of such flat typedisplay apparatus uses, as a light emitting element of a pixel, anelectro-optical element of the current driven type whose emitted lightluminance varies in response to the value of current flowing through theelement. As the electro-optical element of the current driven type, anorganic EL (Electro Luminescence) element is known which utilizes aphenomenon that an organic thin film emits light when an electric fieldis applied thereto.

An organic EL display apparatus which uses an organic EL element as anelectro-optical element of a pixel has the following characteristics. Inparticular, the organic EL element has a low-power consumptioncharacteristic because it can be driven by an application voltage equalto or lower than 10 V. Since the organic EL element is a self luminouselement, the organic EL display apparatus displays an image of highvisibility in comparison with a liquid crystal display apparatus whichdisplays an image by controlling the intensity of light from a lightsource using liquid crystal for each pixel. Besides, since the organicEL element does not require a light source such as a backlight, itfacilitates reduction in weight and thickness of the organic EL displayapparatus. Further, since the speed of response is as high asapproximately several μsec, an after-image upon dynamic picture displaydoes not appear.

The organic EL display apparatus can adopt a simple or passive matrixtype or an active matrix type as a driving method therefor similarly tothe liquid crystal display apparatus. However, although the displayapparatus of the simple matrix type is simple in structure, it has adrawback in that it is difficult to implement the same as a large-sizedhigh definition display apparatus because the light emitting period ofeach electro-optical element decreases as the number of scanning lines,that is, the number of pixels, increases.

Therefore, in recent years, development of an active matrix displayapparatus wherein the current to flow through an electro-optical elementis controlled by an active element provided in a pixel in which theelectro-optical element is provided such as, an insulated gate typefield effect transistor has been and is being carried out vigorously. Asthe insulated gate type field effect transistor, a thin film transistor(TFT) is used popularly. The active matrix display apparatus can beeasily implemented as a large-sized and high definition displayapparatus because the electro-optical element continues to emit lightover a period of one frame.

Incidentally, it is generally known that the I-V characteristic, thatis, the current-voltage characteristic, of the organic EL elementsuffers from deterioration as time passes as known as ageddeterioration. In a pixel circuit which uses a TFT particularly of the Nchannel type as a transistor for driving the organic EL element bycurrent (such transistor is hereinafter referred to as drivingtransistor), if the I-V characteristic of the organic EL element suffersfrom aged deterioration, then the gate-source voltage Vgs of the drivingtransistor varies. As a result, the luminance of emitted light of theorganic EL element varies. This arises from the fact that the organic ELelement is connected to the source electrode side of the drivingtransistor.

This is described more particularly. The source potential of the drivingtransistor depends upon the operating point of the driving transistorand the organic EL element. If the I-V characteristic of the organic ELelement deteriorates, then the operating point of the driving transistorand the organic EL element varies. Therefore, even if the same voltageis applied to the gate electrode of the driving transistor, the sourcepotential of the driving transistor changes. Consequently, thesource-gate voltage Vgs of the driving transistor varies and the valueof current flowing to the driving transistor changes. As a result, sincealso the value of current flowing to the organic EL element varies, theemitted light luminance of the organic EL element varies.

Further, particularly in a pixel circuit which uses a polycrystallinesilicon TFT, in addition to the aged deterioration of the I-Vcharacteristic of the organic EL element, a transistor characteristic ofthe driving transistor varies as time passes or a transistorcharacteristic differs among different pixels due to a dispersion in thefabrication process. In other words, a transistor characteristic of thedriving transistor disperses among individual pixels. The transistorcharacteristic may be a threshold voltage Vth of the driving transistor,the mobility μ of a semiconductor thin film which forms the channel ofthe driving transistor (such mobility μ is hereinafter referred tosimply as “mobility μ of the driving transistor”) or some othercharacteristic.

Where a transistor characteristic of the driving transistor differsamong different pixels, since this gives rise to a dispersion of thevalue of current flowing to the driving transistor among the pixels,even if the same voltage is applied to the gate electrode of the drivingtransistor among the pixels, a dispersion appears in the emitted lightluminance of the organic EL element among the pixels. As a result, theuniformity of the screen image is damaged.

Therefore, various correction or compensation functions are provided toa pixel circuit in order to keep the emitted light luminance of theorganic EL element fixed without being influenced by aged deteriorationof the I-V characteristic of the organic EL element or ageddeterioration of a transistor characteristic of the driving transistoras disclosed, for example, in Japanese Patent Laid-Open No. 2007-310311.

The correction functions may include a compensation function for avariation of the I-V characteristic variation of the organic EL element,a correction function against the variation of the threshold voltage Vthof the driving transistor, a correction function against the variationof the mobility μ of the driving transistor and some other function. Inthe description given below, the correction against the variation of thethreshold voltage Vth of the driving transistor is referred to as“threshold value correction,” and the correction against the mobility μof the driving transistor is referred to as “mobility correction.”

Where each pixel circuit is provided with various correction functionsin this manner, the emitted light luminance of the organic EL elementcan be kept fixed without being influenced by aged deterioration of theI-V characteristic of the organic EL element or aged deterioration of atransistor characteristic of the driving transistor. As a result, thedisplay quality of the organic EL display apparatus can be improved.

SUMMARY OF THE INVENTION

Incidentally, if light having high energy is inputted to the channel ofa transistor in a pixel in a state wherein a certain fixed voltage isapplied to the transistor in the pixel, then the threshold voltage ofthe transistor shifts to the negative side. In particular, if blue lighthaving a relatively short wavelength and hence having high energy isinputted to the transistor, then the characteristic shift of thetransistors becomes very great in comparison with that when no light isinputted as seen from FIG. 26.

As an example, subpixels where subpixel units each including threesubpixels for R (red), G (green) and B (blue) are disposed such that theB subpixel is positioned at the center in each unit are considered. TheB subpixel is influenced by blue light only when the B subpixel itselfemits light.

However, since the R and G subpixels are positioned adjacent the Bsubpixel, even if they themselves do not emit light, they are influencedby light emitted from the B subpixel positioned adjacent thereto. Wherethe R and G subpixels are influenced not only by light emitted from themthemselves but also by light emitted from the adjacent subpixel, it isvery difficult to compensate for a current variation in a correctionprocess or a like process.

Although the foregoing description is given in regard to a B subpixel incolor coating of RGB, this similarly applies also to a subpixel whichemits light of the shortest wavelength and hence of the highest energyin any other color coating.

Accordingly, it is demanded to provide a display apparatus and anelectronic apparatus wherein it is possible to suppress a characteristicshift of a transistor of a subpixel which appears when light having highenergy is inputted to the channel of the transistor.

According to an embodiment of the present invention, there is provided adisplay apparatus including a plurality of subpixels disposed adjacenteach other and forming one pixel which forms a unit for formation of acolor image, the plurality of subpixels including a first subpixel whichemits light of the shortest wavelength and a second subpixel disposedadjacent the first subpixel, the second subpixel having a light blockingmember disposed between the second subpixel and the first subpixel andhaving a width greater than a channel length or a channel width of atransistor which forms the second subpixel.

Emitted light having a relatively short wavelength has high energy. Inthe display apparatus, if light having the shortest wavelength andhaving high intensity is inputted from the first subpixel to the channelof the transistor in the second subpixel which is positioned adjacentthe first subpixel and to which a voltage is applied, then acharacteristic shift occurs with the transistor. Here, since the secondsubpixel has the light blocking member disposed between the secondsubpixel and the first subpixel, the light blocking member acts to blockthe light emitted from the first subpixel from entering the secondsubpixel. Consequently, the characteristic shift of the transistor whichis caused by incidence of light having high energy to the channel of thetransistor can be suppressed.

With the display apparatus, since a characteristic shift of thetransistor which is caused by incidence of light having high energy tothe channel of the transistor can be suppressed, decrease of current toflow to an electro-optical element and occurrence of a fault in picturequality such as stripes and luminance unevenness can be suppressed.

The above and other features and advantages of the embodiment of thepresent invention will become apparent from the following descriptionand the appended claims, taken in conjunction with the accompanyingdrawings in which like parts or elements denoted by like referencesymbols.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a general system configuration of anorganic EL display apparatus to which an embodiment of the presentinvention is applied;

FIG. 2 is a block circuit diagram showing a circuit configuration of apixel;

FIG. 3 is a sectional view showing an example of a sectional structureof a pixel;

FIG. 4 is a timing waveform diagram illustrating circuit operation ofthe organic EL display apparatus of FIG. 1;

FIGS. 5A to 5D and 6A to 6D are circuit diagrams illustrating circuitoperations of the organic EL display apparatus of FIG. 1;

FIG. 7 is a characteristic diagram illustrating a subject to be solvedwhich arises from a dispersion of a threshold voltage of a drivingtransistor;

FIG. 8 is a characteristic diagram illustrating a subject to be solvedwhich arises from a dispersion of a mobility of the driving transistor;

FIGS. 9A to 9C are characteristic diagrams illustrating relationshipsbetween a signal voltage of an image signal and drain-source current ofthe driving transistor depending upon whether or not threshold valuecorrection and/or mobility correction are carried out;

FIG. 10 is an equivalent circuit diagram illustrating a potentialrelationship of the electrodes of a writing transistor when the white isdisplayed;

FIG. 11 is a sectional view showing an example of a sectional structureof the writing transistor;

FIG. 12 is a waveform diagram illustrating a transition waveform of awriting scanning signal in a state wherein it is deformed at a risingedge and a falling edge thereof;

FIG. 13 is a sectional view of a pixel section illustrating an exampleof a method of preventing a pixel from being influenced by blue lightfrom an adjacent pixel;

FIG. 14 is a plan view showing a light blocking layout structureaccording to a working example 1;

FIG. 15 is a sectional view taken along line A-A′ of FIG. 14 showing asectional structure of the light block layout structure;

FIGS. 16 and 17 are plan views showing light blocking layout structuresaccording to modifications 1 and 2 to the working example 1 of FIG. 14;

FIGS. 18 and 19 are sectional views showing light blocking layoutstructures according to modifications 3 and 4 to the working example 1of FIG. 14;

FIG. 20 is a sectional view showing a light blocking layout structureaccording to a working example 2;

FIG. 21 is a perspective view showing an appearance of a television setto which an embodiment of the present invention is applied;

FIGS. 22A and 22B are perspective views showing an appearance of adigital camera to which an embodiment of the present invention isapplied as viewed from the front side and the rear side, respectively;

FIG. 23 is a perspective view showing an appearance of a notebook typepersonal computer to which an embodiment of the present invention isapplied;

FIG. 24 is a perspective view showing an appearance of a video camera towhich an embodiment of the present invention is applied;

FIGS. 25A and 25B are a front elevational view and a side elevationalview showing an appearance of a portable telephone set to which anembodiment of the present invention is applied in an unfolded state andFIGS. 25C, 25D, 25E, 25F and 25G are a front elevational view, a leftside elevational view, a right side elevational view, a top plan viewand a bottom plan view of the portable telephone set in a folded state,respectively; and

FIG. 26 is a diagrammatic view of a transistor characteristicillustrating a manner wherein a characteristic of a transistor varies bya great amount when blue light is inputted to the channel of thetransistor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following, a preferred embodiment of the present invention isdescribed in detail with reference to the accompanying drawings. It isto be noted that the description is given in the following order:

1. Embodiment (light blocking layout of the pixels)1-1. Working example 11-2. Working example 2

2. Modifications

3. Applications (electronic apparatus)

1. Embodiment System Configuration

FIG. 1 is a block diagram showing a general system configuration of anactive matrix display apparatus to which an embodiment of the presentinvention is applied. Here, it is assumed that the active matrix displayapparatus described is an active matrix organic EL display apparatuswherein an organic EL element which is an electro-optical element of thecurrent driven type whose emitted light luminance varies in response thevalue of current flowing through the element is used as a light emittingelement of a pixel or pixel circuit.

Referring to FIG. 1, the organic EL display apparatus 10 shown includesa plurality of pixels 20 each including a light emitting element, apixel array section 30 in which the pixels 20 are arrangedtwo-dimensionally in rows and columns, that is, in a matrix, and drivingsections disposed around the pixel array section 30. The drivingsections drive the pixels 20 of the pixel array section 30 to emitlight.

The driving sections for the pixels 20 include a scanning driving systemincluding a writing scanning circuit 40 and a power supply scanningcircuit 50 and a signal supplying system including a signal outputtingcircuit 60. In the organic EL display apparatus 10 of the presentembodiment, the signal outputting circuit 60 is provided on a displaypanel 70 on which the pixel array section 30 is formed while the writingscanning circuit 40 and the power supply scanning circuit 50 which formthe scanning driving system are provided externally of the display panelor substrate 70.

Here, if the organic EL display apparatus 10 is ready for white/blackdisplay, then one pixel which makes a unit for forming a monochromaticimage corresponds to a pixel 20. On the other hand, where the organic ELdisplay apparatus 10 is ready for color display, one pixel which makes aunit for forming a color image is formed from a plurality of subpixels,each of which corresponds to a pixel 20. More particularly, in a displayapparatus for color display, one pixel is composed of three subpixelsincluding a subpixel for emitting red light (R), another subpixel foremitting green light (G) and a further subpixel for emitting blue right(B).

However, one pixel is not necessarily formed from a combination ofsubpixels of the three primary colors of R, G and B but may be formedfrom one or a plurality of subpixels of a color or different colors inaddition to the subpixels of the three primary colors. In particular,for example, a subpixel for emitting white light (W) may be added toform one pixel in order to raise the luminance, or at least one subpixelfor emitting light of a complementary color may be added to form onepixel in order to expand the color reproduction range.

The pixels 20 are arrayed in m rows and n columns in the pixel arraysection 30, and scanning lines 31-1 to 31-m and power supply lines 32-1to 32-m are wired for the individual pixel rows along the direction of arow, that is, along the direction along which the pixels in a pixel roware arranged. Further, signal lines 33-1 to 33-n are wired for theindividual pixel columns along the direction of a column, that is, alongthe direction along which the pixels in a pixel column are arranged.

The scanning lines 31-1 to 31-m are individually connected to outputterminals of the writing scanning circuit 40 for the corresponding rows.The power supply lines 32-1 to 32-m are individually connected to outputterminals of the power supply scanning circuit 50 for the correspondingrows. The signal lines 33-1 to 33-n are individually connected to outputterminals of the signal outputting circuit 60 for the correspondingcolumns.

The pixel array section 30 is normally formed on a transparentinsulating substrate such as a glass substrate. Consequently, theorganic EL display apparatus 10 has a flat panel structure. A drivingcircuit for each of the pixels 20 of the pixel array section 30 can beformed using an amorphous silicon TFT (Thin Film Transistor) or a lowtemperature polycrystalline silicon TFT. Where a low temperaturepolycrystalline silicon TFT is used, also the writing scanning circuit40 and power supply scanning circuit 50 can be mounted on the displaypanel or substrate 70.

The writing scanning circuit 40 is formed from a shift register whichsuccessively shifts a start pulse sp in synchronism with a clock pulseck or from a like element. Upon writing of an image signal into thepixels 20 in the pixel array section 30, the writing scanning circuit 40successively supplies a writing scanning signal WS (WS1 to WSm) to thescanning lines 31-1 to 31-m to successively scan (line sequentialscanning) the pixels 20 of the pixel array section 30 in a unit of arow.

The power supply scanning circuit 50 is formed from a shift registerwhich successively shifts the start pulse sp in synchronism with theclock pulse ck or from a like element. The power supply scanning circuit50 supplies a power supply potential DS (DS1 to DSm), which changes overbetween a first power supply potential Vccp and a second power supplypotential Vini which is lower than the first power supply potentialVccp, to the power supply lines 32-1 to 32-m in synchronism with linesequential scanning by the writing scanning circuit 40. By thechangeover of the power supply potential DS between the first powersupply potential Vccp and the second power supply potential Vini,control of light emission/no-light emission of the pixels 20 is carriedout.

The signal outputting circuit 60 selects one of a signal voltage Vsig ofan image signal supplied from a signal supply line not shown andrepresentative of luminance information and a reference potential Vofsand outputs the selected voltage. The reference potential Vofsselectively outputted from the signal outputting circuit 60 is used as areference for the signal voltage Vsig of the image signal andcorresponds, for example, to the black level of the image signal.

The signal outputting circuit 60 may be formed using a well-knowncircuit configuration, for example, of a time-division driving system.The time-division driving system is also called selector system andallocates a plurality of signal lines in a unit or group to one ofoutput terminals of a driver (not shown) which serves as a signalsupplying source. Then, the plural signal lines are successivelyselected time-divisionally, and image signals outputted in a time seriesfor the individual output terminals of the driver are distributed andsupplied time divisionally to the selected signal lines to drive thesignal lines.

In the case of a display apparatus ready for color display as anexample, image signals of R, G and B are inputted in a time series tothe signal outputting circuit 60 within one horizontal period from thedriver in a unit of three pixels of R, G and B which are positionedadjacent. The signal outputting circuit 60 is formed from selectors orselection switches provided corresponding to the three pixel columns ofR, G and B such that the selectors successively carry out a turning onoperation time-divisionally to write image signals of R, G and Btime-divisionally into corresponding signal lines.

While one unit here includes three pixel columns or signal lines of R, Gand B, the unit is not limited to this. In particular, since thetime-division driving method or selector method is adopted, where thetime-division number is represented by x which is an integer equal to orgreater than 2, the number of outputs of the driver and the number oflines between the driver and the signal outputting circuit 60 and hencebetween the driver and the display panel 70 can be reduced to 1/x thenumber of signal lines.

The signal voltage Vsig or the reference potential Vofs selectivelyoutputted from the signal outputting circuit 60 is written into thepixels 20 of the pixel array section 30 in a unit of a row through thesignal lines 33-1 to 33-n. In other words, the signal outputting circuit60 exhibits a line sequential writing driving form wherein the signalvoltage Vsig is written in a unit of a row or line.

(Pixel Circuit)

FIG. 2 shows a particular circuit configuration of a pixel or pixelcircuit 20 used in the organic EL display apparatus 10 according to thepresent embodiment.

Referring to FIG. 2, the pixel 20 includes an electro-optical element ofthe current driven type whose emitted light luminance varies in responseto the value of current flowing therethrough such as, an organic ELelement 21, and a driving circuit for driving the organic EL element 21.The organic EL element 21 is connected at the cathode electrode thereofto a common power supply line 34 which is wired commonly to all pixels20.

The driving circuit for driving the organic EL element 21 includes adriving transistor 22, a writing transistor or sampling transistor 23,and a storage capacitor 24. Here, an N-channel TFT is used for thedriving transistor 22 and the writing transistor 23. However, thiscombination of the conduction types of the driving transistor 22 and thewriting transistor 23 is a mere example, and the combination of suchconduction types is not limited to this specific combination.

It is to be noted that, where an N-channel TFT is used for the drivingtransistor 22 and the writing transistor 23, an amorphous silicon (a-Si)process can be used for the fabrication of them. Where the a-Si processis used, reduction of the cost of a substrate on which the TFTs are tobe produced and reduction of the cost of the organic EL displayapparatus 10 can be anticipated. Further, if the driving transistor 22and the writing transistor 23 are formed in a combination of the sameconduction type, then since the transistors 22 and 23 can be produced bythe same process, this can contribute to reduction of the cost.

The driving transistor 22 is connected at a first electrode thereof,that is, at the source/drain electrode thereof, to the anode electrodeof the organic EL element 21 and at a second electrode thereof, that is,at the drain/source electrode thereof, to a power supply line 32 (32-1to 32-m).

The writing transistor 23 is connected at the gate electrode thereof toa scanning line 31 (31-1 to 31-m). Further, the writing transistor 23 isconnected at a first electrode thereof, that is, at the source/drainelectrode thereof, to a signal line 33 (33-1 to 33-n) and at a secondelectrode thereof, that is, at the drain/source electrode thereof, tothe gate electrode of the driving transistor 22.

In the driving transistor 22 and the writing transistor 23, the firstelectrode is a metal line electrically connected to the source/drainregion, and the second electrode is a metal line electrically connectedto the drain/source region. Further, depending upon the relationship ofthe potential between the first electrode and the second electrode, thefirst electrode may be the source electrode or the drain electrode, andthe second electrode may be the drain electrode or the source electrode.

The storage capacitor 24 is connected at an electrode thereof to thegate electrode of the driving transistor 22 and at the other electrodethereof to the second electrode of the driving transistor 22 and theanode electrode of the organic EL element 21.

It is to be noted that the circuit configuration of the driving circuitfor the organic EL element 21 is not limited to that which includes thetwo transistors of the driving transistor 22 and the writing transistor23 and the one capacitor element of the storage capacitor 24. Forexample, it is possible to adopt another circuit configuration whereinan auxiliary capacitor connected at an electrode thereof to the anodeelectrode of the organic EL element 21 and at the other electrodethereof to a fixed potential is provided as occasion demands in order tomake up for shortage of the capacitance of the organic EL element 21.

In the pixel 20 having the configuration described above, the writingtransistor 23 is placed into a conducting state in response to aHigh-active writing scanning signal WS applied to the gate electrode ofthe writing transistor 23 through the scanning line 31 from the writingscanning circuit 40. Consequently, the writing transistor 23 samples thesignal voltage Vsig of an image signal representative of luminanceinformation or the reference potential Vofs supplied from the signaloutputting circuit 60 through the signal line 33 and writes the sampledpotential into the pixel 20. The thus written signal voltage Vsig orreference potential Vofs is applied to the gate electrode of the drivingtransistor 22 and stored into the storage capacitor 24.

The driving transistor 22 operates, when the power supply potential DSof the power supply line 32 (32-1 to 32-m) is the first power supplypotential Vccp, in a saturation region while the first electrode servesas the drain electrode and the second electrode serves as the sourceelectrode. Consequently, the driving transistor 22 receives supply ofcurrent from the power supply line 32 and drives the organic EL element21 by current driving to emit light. More particularly, the drivingtransistor 22 operates in a saturation region thereof to supply drivingcurrent of a current value corresponding to the voltage value of thesignal voltage Vsig stored in the storage capacitor 24 to the organic ELelement 21 to drive the organic EL element 21 with the current so as toemit light.

Further, when the power supply potential DS changes over from the firstpower supply potential Vccp to the second power supply potential Vini,the first electrode of the driving transistor 22 serves as the sourceelectrode while the second electrode of the driving transistor 22 servesas the drain electrode, and the driving transistor 22 operates as aswitching transistor. Consequently, the driving transistor 22 stopssupply of driving current to the organic EL element 21 by switchingoperation thereof to place the organic EL element 21 into a no-lightemitting state. Thus, the driving transistor 22 has a function also as atransistor for controlling light emission/no-light mission of theorganic EL element 21.

The switching operation of the driving transistor 22 provides a periodwithin which the organic EL element 21 is in a no-light emitting state,that is, a no-light emitting period and controls the ratio between thelight emitting period and the no-light emitting period of the organic ELelement 21, that is, the duty of the organic EL element 21. By this dutycontrol, after-image blurring caused by emission of light from a pixel20 over a one-frame period can be reduced, and consequently, the picturequality particularly of a dynamic picture can be enhanced.

The first power supply potential Vccp from between the first and secondpower supply potentials Vccp and Vini selectively supplied from thepower supply scanning circuit 50 through the power supply line 32 is apower supply potential for supplying driving current for driving theorganic EL element 21 to emit light to the organic EL element 21.Meanwhile, the second power supply potential Vini is used to apply areverse bias to the organic EL element 21. This second power supplypotential Vini is set to a potential lower than the reference potentialVofs for the signal voltage, for example, to a potential lower thanVofs−Vth where Vth is a threshold voltage of the driving transistor 22,preferably to a potential sufficiently lower than Vofs−Vth.

(Pixel Structure)

FIG. 3 shows a sectional structure of a pixel 20. Referring to FIG. 3,the pixel 20 is formed on a glass substrate 201 on which a drivingcircuit including a driving transistor 22 and so forth is formed. Thepixel 20 is configured such that an insulating film 202, an insulatingflattening film 203 and a window insulating film 204 are formed in orderon the glass substrate 201 and an organic EL element 21 is provided at arecessed portion 204A of the window insulating film 204. Here, fromamong the components of the driving circuit, only the driving transistor22 is shown while the other components are omitted.

The organic EL element 21 is formed from an anode electrode 205 made ofmetal or the like, an organic layer 206 formed on the anode electrode205, and a cathode electrode 207 formed from a transparent conductivefilm or the like formed commonly to all pixels on the organic layer 206.The anode electrode 205 is formed on the bottom of the recessed portion204A of the window insulating film 204.

In the organic EL element 21, the organic layer 206 is formed from ahole transport layer/hole injection layer 2061, a light emitting layer2062, an electron transport layer 2063 and an electron injection layer(not shown) deposited in order on the anode electrode 205. If currentflows from the driving transistor 22 to the organic layer 206 throughthe anode electrode 205 under the current driving by the drivingtransistor 22 shown in FIG. 2, then electrons and holes are recombinedin the light emitting layer 2062 in the organic layer 206, whereuponlight is emitted from the light emitting layer 2062.

The driving transistor 22 includes a gate electrode 221, a channelformation region 225 provided at a portion of the semiconductor layer222 opposing to the gate electrode 221 and source/drain regions 223 and224 provided on the opposite sides of the channel formation region 225on a semiconductor layer 222. The source/drain region 223 iselectrically connected to the anode electrode 205 of the organic ELelement 21 through a contact hole.

Then, the organic EL element 21 is formed in a unit of a pixel on theglass substrate 201, on which the driving circuit including the drivingtransistor 22 is formed, through the insulating film 202, insulatingflattening film 203 and window insulating film 204. Then, a sealingsubstrate 209 is adhered through a passivation film 208 by a bondingagent 210, whereupon the organic EL element 21 is sealed with thesealing substrate 209 to form the display panel 70.

[Circuit Operation of the Organic EL Display Apparatus]

Now, circuit operation of the organic EL display apparatus 10 whereinthe pixels 20 having the configuration described above are arrangedtwo-dimensionally is described with reference to FIGS. 5A to 5D and 6Ato 6D in addition to FIG. 4.

It is to be noted that, in FIGS. 5A to 5D and 6A to 6D, the writingtransistor 23 is represented by a symbol of a switch for simplifiedillustration. Further, as well known in the art, the organic EL element21 has equivalent capacitance or parasitic capacitance Cel. Accordingly,also the equivalent capacitance Cel is shown in FIGS. 5A to 5D and 6A to6D.

In FIG. 4, a variation of the potential of the writing scanning signalWS of a scanning line 31 (31-1 to 31-m), a variation of the potential ofthe power supply potential DS of a power supply line 32 (32-1 to 32-m)and variations of the gate potential Vg and the source potential Vs ofthe driving transistor 22.

<<Light Emitting Period within the Preceding Frame>>

In FIG. 4, prior to time t1, a light emitting period of the organic ELelement 21 within the preceding frame or field is provided. Within thelight emitting period of the preceding frame, the power supply potentialDS of the power supply line 32 has a first power supply potential(hereinafter referred to as “high potential”) Vccp and the writingtransistor 23 is in a non-conductive state.

The driving transistor 22 is designed such that, at this time, itoperates in a saturation region. Consequently, driving current ordrain-source current Ids corresponding to the gate-source voltage Vgs ofthe driving transistor 22 is supplied from the power supply line 32 tothe organic. EL element 21 through the driving transistor 22.Consequently, the organic EL element 21 emits light with a luminancecorresponding to the current value of the driving current Ids.

<<Threshold Value Correction Preparation Period>>

At time t1, a new frame of line sequential scanning, that is, a currentframe, is entered. Then, the potential DS of the power supply line 32changes over from the high potential Vccp to a second power supplyvoltage (hereinafter referred to as “low potential”) Vini, which issufficiently lower than Vofs−Vth, with respect to the referencepotential Vofs of the signal line 33 as seen from FIG. 5B.

Here, the threshold voltage of the organic EL element 21 is representedby Vthe1, and the potential of the common power supply line 34, that is,the cathode potential, is represented by Vcath. At this time, if the lowpotential Vini satisfies Vini<Vthe1+Vcath, then since the sourcepotential Vs of the driving transistor 22 becomes substantially equal tothe low potential Vini, the organic EL element 21 is placed into areversely biased state and stops the emission of light.

Then, when the potential WS of the scanning line 31 changes from the lowpotential side to the high potential side at time t2, the writingtransistor 23 is placed into a conducting state as seen from FIG. 5C. Atthis time, since the reference potential Vofs is supplied from thesignal outputting circuit 60 to the signal line 33, the gate potentialVg of the driving transistor 22 becomes equal to the reference potentialVofs. Meanwhile, the source potential Vs of the driving transistor 22 isequal to the low potential Vini sufficiently lower than the referencepotential Vofs.

At this time, the gate-source voltage Vgs of the driving transistor 22is Vofs−Vini. Here, if Vofs−Vini is not sufficiently greater than thethreshold potential Vth of the driving transistor 22, then a thresholdvalue correction process hereinafter described cannot be carried out,and therefore, it is necessary to establish the potential relationshipof Vofs−Vini>Vth.

In this manner, the process of fixing or finalizing the gate potentialVg of the driving transistor 22 to the reference potential Vofs and thesource potential Vs of the driving transistor 22 to the low potentialVini to initialize them is a process of preparation (threshold valuecorrection preparation) before a threshold value correction processhereinafter described is carried out. Accordingly, the referencepotential Vofs and the low potential Vini become initializationpotentials for the gate potential Vg and the source potential Vs of thedriving transistor 22, respectively.

<<Threshold Value Correction Period>>

Then, if the potential DS of the power supply line 32 changes over fromthe low potential Vini to the high potential Vccp at time t3 as seen inFIG. 5D, then a threshold value correction process is started in a statewherein the gate potential Vg of the driving transistor 22 ismaintained. In particular, the source potential Vs of the drivingtransistor 22 begins to rise toward the potential of the difference ofthe threshold potential Vth of the driving transistor 22 from the gatepotential Vg.

Here, the process of varying the source potential Vs toward thepotential of the difference of the threshold potential Vth of thedriving transistor 22 from the reference potential Vofs with referenceto the initialization potential Vofs at the gate electrode of thedriving transistor 22 is hereinafter referred to as threshold valuecorrection process. As the threshold value correction processprogresses, the gate-source voltage Vgs of the driving transistor 22soon converges to the threshold potential Vth of the driving transistor22. The voltage corresponding to the threshold potential Vth is storedinto the storage capacitor 24.

It is to be noted that it is necessary to allow, within a period withinwhich the threshold value correction process is carried out, that is,within a threshold value correction period, current to wholly flow tothe storage capacitor 24 side but not to flow to the organic EL element21 side. To this end, the potential Vcath of the common power supplyline 34 is set so that the organic EL element 21 has a cutoff state.

Then, the potential WS of the scanning line 31 changes to the lowpotential side at time t4, whereupon the writing transistor 23 is placedinto a non-conducting state as seen in FIG. 6A. At this time, the gateelectrode of the driving transistor 22 is electrically disconnected fromthe signal line 33 and enters a floating state. However, since thegate-source voltage Vgs is equal to the threshold potential Vth of thedriving transistor 22, the driving transistor 22 remains in a cutoffstate. Accordingly, the amount of the drain-source current Ids flowingto the driving transistor 22 is very small.

<<Signal Writing & Mobility Correction Period>>

Then at time t5, the potential of the signal line 33 changes over fromthe reference potential Vofs to the signal voltage Vsig of the imagesignal as seen in FIG. 6B. Then at time t6, the potential WS of thescanning line 31 changes to the high potential side, whereupon thewriting transistor 23 is placed into a conducting state as seen in FIG.6C to sample and write the signal voltage Vsig of the image signal intothe pixel 20.

By the writing of the signal voltage Vsig by the writing transistor 23,the gate potential Vg of the driving transistor 22 becomes equal to thesignal voltage Vsig. Then, upon driving of the driving transistor 22with the signal voltage Vsig of the image signal, the thresholdpotential Vth of the driving transistor 22 is canceled with the voltagecorresponding to the threshold potential Vth stored in the storagecapacitor 24. Details of the principle of the threshold valuecancellation are hereinafter described in detail.

At this time, the organic EL element 21 remains in a cutoff state, thatis, in a high-impedance state. Accordingly, current flowing from thepower supply line 32 to the driving transistor 22 in response to thesignal voltage Vsig of the image signal, that is, the drain-sourcecurrent Ids, flows into the equivalent capacitance Cel. Charging of theequivalent capacitance Cel of the organic EL element 21 is started.

By the charging of the equivalent capacitance Cel, the source potentialVs of the driving transistor 22 rises together with lapse of time. Atthis time, a dispersion of the threshold potential Vth of the drivingtransistor 22 for each pixel is canceled already, and the drain-sourcecurrent Ids of the driving transistor 22 exhibits a value which reliesupon the mobility μ of the driving transistor 22.

Here, it is assumed that the ratio of the storage voltage Vgs of thestorage capacitor 24 to the signal voltage Vsig of the image signal is1, which is an ideal value. The ratio of the storage voltage Vgs to thesignal voltage Vsig is hereinafter referred to sometimes as write gain.In this instance, when the source potential Vs of the driving transistor22 rises to the potential of Vofs−Vth+ΔV, the gate-source voltage Vgs ofthe driving transistor 22 becomes Vsig−Vofs+Vth−ΔV.

In particular, the rise amount ΔV of the source potential Vs of thedriving transistor 22 acts so as to be subtracted from the voltagestored in the storage capacitor 24, that is, from Vsig−Vofs+Vth. Or inother words, the rise amount ΔV of the source potential Vs acts so as todischarge the accumulated charge of the storage capacitor 24, andtherefore, is negatively fed back. Accordingly, the rise amount ΔV ofthe source potential Vs of the driving transistor 22 is a feedbackamount in the negative feedback.

By applying negative feedback of the feedback amount ΔV in accordancewith the driving current Ids flowing through the driving transistor 22to the gate-source voltage Vgs in this manner, the dependency of thedrains-source current Ids of the driving transistor 22 upon the mobilityμ can be canceled. This process of canceling the dependency upon themobility μ is a mobility correction process of correcting the dispersionof the mobility μ of the driving transistor 22 for each pixel.

More particularly, since the drain-source current Ids increases as thesignal amplitude Vin (=Vsig−Vofs) of the image signal to be written intothe gate electrode of the driving transistor 22 increases, also theabsolute value of the feedback amount ΔV of the negative feedbackincreases. Accordingly, a mobility correction process in accordance withthe emitted light luminance level is carried out.

Further, if it is assumed that the signal amplitude Vin of the imagesignal is fixed, then since also the absolute value of the feedbackamount ΔV of the negative feedback increases as the mobility μ of thedriving transistor 22 increases, a dispersion of the mobility μ for eachpixel can be removed. Accordingly, the feedback amount ΔV of thenegative feedback can be regarded also as a correction amount ofmobility correction. Details of the principle of the mobility correctionare hereafter described.

<<Light Emitting Period>>

Then, the potential WS of the scanning line 31 changes to the lowpotential side at time t7, whereupon the writing transistor 23 is placedinto a non-conducting state as seen from FIG. 6D. Consequently, the gatepotential of the driving transistor 22 is placed into a floating statebecause it is electrically disconnected from the signal line 33.

Here, when the gate electrode of the driving transistor 22 is in afloating state, since the storage capacitor 24 is connected between thegate and the source of the driving transistor 22, also the gatepotential Vg varies in an interlocked relationship or following uprelationship with a variation of the source potential Vs of the drivingtransistor 22. An operation of the gate potential Vg of the drivingtransistor 22 which varies in an interlocked relationship with avariation of the source potential Vs in this manner is hereinafterreferred to as bootstrap operation by the storage capacitor 24.

When the gate electrode of the driving transistor 22 is placed into afloating state and the drain-source current Ids of the drivingtransistor 22 simultaneously begins to flow to the organic EL element21, the anode potential of the organic EL element 21 rises in responseto the drain-source current Ids.

Then, when the anode potential of the organic EL element 21 exceedsVthe1+Vcath, driving current begins to flow to the organic EL element21, and consequently, the organic EL element 21 starts emission oflight. Further, the rise of the anode potential of the organic ELelement 21 is nothing but a rise of the source potential Vs of thedriving transistor 22. As the source potential Vs of the drivingtransistor 22 rises, also the gate potential Vg of the drivingtransistor 22 rises in an interlinked relationship by the bootstrapoperation of the storage capacitor 24.

At this time, if it is assumed that the bootstrap gain is 1 in an idealstate, then the rise amount of the gate potential Vg is equal to therise amount of the source potential Vs. Therefore, during the lightemitting period, the gate-source voltage Vgs of the driving transistor22 is kept fixed at Vsig−Vofs+Vth−ΔV. Then, at time t8, the potential ofthe signal line 33 changes over from the signal voltage Vsig of theimage signal to the reference potential Vofs.

In a series of circuit operations described above, the processingoperations of threshold value correction preparation, threshold valuecorrection, writing of the signal voltage Vsig (signal writing) andmobility correction are executed within one horizontal scanning period(1H). Meanwhile, the processing operations of signal writing andmobility correction are executed in parallel within the period from timet6 to time t7.

It is to be noted here that, although the example described above adoptsthe driving method wherein the threshold value correction process isexecuted only once, this driving method is a mere example and thedriving method to be adopted is not limited to this. For example, it ispossible to adopt a driving method wherein the threshold valuecorrection process is executed within a 1H period within which it iscarried out together with the mobility correction and signal writingprocesses and is further executed by a plural number of timesdivisionally in a plurality of horizontal scanning periods preceding tothe 1H period.

Where the driving method of the divisional threshold value correctionjust described is adopted, even if the time allocated to one horizontalscanning period is decreased by increase of the number of pixels byincrease of the definition, a sufficient period of time can be assuredas the threshold value correction period over a plurality of horizontalscanning periods. Consequently, the threshold value correction processcan be carried out with certainty.

(Principle of the Threshold Value Cancellation)

Here, the principle of threshold value cancellation, that is, of thethreshold value correction, by the driving transistor 22 is described.The threshold value correction process is a process of varying thesource voltage Vs of the driving transistor 22 toward a potential of thedifference of the threshold voltage Vth of the driving transistor 22from the initialization potential Vofs for the gate potential Vg of thedriving transistor 22 with reference to the initialization potentialVofs as described hereinabove.

The driving transistor 22 operates as a constant current source becauseit is designed so as to operate in a saturation region. Since thedriving transistor 22 operates as a constant current source, the organicEL element 21 is supplied with fixed drain-source current or drivingcurrent Ids given by the following expression (1):

Ids=(½)·μ(W/L)Cox(Vgs−Vth)²  (1)

where W is the channel width of the driving transistor 22, L the channellength, and Cox the gate capacitance per unit area.

FIG. 7 illustrates a characteristic of the drain-source current Ids withrespect to the gate-source voltage Vgs of the driving transistor 22.

As seen from the characteristic diagram of FIG. 7, if a cancellationprocess for a dispersion of the threshold potential Vth of the drivingtransistor 22 for each pixel is not carried out, then when the thresholdpotential Vth is Vth1, the drain-source current Ids corresponding to thegate-source potential Vgs becomes Ids1.

In contrast, when the threshold potential Vth is Vth2 (Vth2>Vth1), thedrain-source current Ids corresponding to the same gate-source voltageVgs becomes Ids2 (Ids2<Ids1). In other words, if the threshold potentialVth of the driving transistor 22 fluctuates, then even if thegate-source voltage Vgs is fixed, the drain-source current Idsfluctuates.

On the other hand, in the pixel or pixel circuit 20 having theconfiguration described above, the gate-source voltage Vgs of thedriving transistor 22 upon light emission is Vsig−Vofs+Vth−ΔV.Accordingly, by substituting this into the expression (1), thedrain-source current Ids is represented by the following expression (2):

Ids=(½)·μ(W/L)Cox(Vsig−Vofs−ΔV)²  (2)

In particular, the term of the threshold potential Vth of the drivingtransistor 22 is canceled, and the drain-source current Ids to besupplied from the driving transistor 22 to the organic EL element 21does not rely upon the threshold potential Vth of the driving transistor22. As a result, even if the threshold potential Vth of the drivingtransistor 22 varies for each pixel due to a dispersion of thefabrication process or aged deterioration of the driving transistor 22,the drain-source current Ids does not vary, and consequently, theemitted light luminance of the organic EL element 21 can be kept fixed.

(Principle of the Mobility Correction)

Now, the principle of the mobility correction of the driving transistor22 is described. The mobility correction process is a process ofapplying negative feedback of the correction amount ΔV corresponding tothe drain-source current Ids flowing to the driving transistor 22 to thepotential difference between the gate and the source of the drivingtransistor 22. By the mobility correction process, the dependency of thedrain-source current Ids of the driving transistor 22 upon the mobilityμ can be canceled.

FIG. 8 illustrates characteristic curves of a pixel A whose drivingtransistor 22 has a relatively high mobility μ and a pixel B whosedriving transistor 22 has a relatively low mobility μ for comparison.Where the driving transistor 22 is formed from a polycrystalline siliconthin film transistor or the like, it cannot be avoided that the mobilityμ disperses among pixels like the pixel A and the pixel B.

It is assumed here that, in a state wherein the pixel A and the pixel Bhave a dispersion in mobility μ therebetween, the signal amplitudes Vin(=Vsig−Vofs) of an equal level are written into the gate electrodes ofthe driving transistors 22 in the pixels A and B. In this instance, ifcorrection of the mobility μ is not carried out at all, then a greatdifference appears between the drain-source current Ids1′ flowingthrough the pixel A having the high mobility μ and the drain-sourcecurrent Ids2′ flowing through the pixel B having the low mobility μ. Ifa great difference in the drain-source current Ids appears betweendifferent pixels originating from the dispersion of the mobility μ amongthe pixels in this manner, then uniformity of the screen image isdamaged.

Here, as apparent from the transistor characteristic expression of theexpression (1) given hereinabove, where the mobility μ is high, thedrain-source current Ids is great. Accordingly, the feedback amount ΔVin the negative feedback increases as the mobility μ increases. As seenfrom FIG. 8, the feedback amount ΔV1 in the pixel A of the high mobilityμ is greater than the feedback amount ΔV2 in the pixel B having the lowmobility μ.

Therefore, if negative feedback is applied to the gate-source voltageVgs with the feedback amount ΔV in accordance with the drain-sourcecurrent Ids of the driving transistor 22 by the mobility correctionprocess, then the negative feedback amount increases as the mobility μincreases. As a result, the dispersion of the mobility μ among thepixels can be suppressed.

In particular, if correction of the feedback amount ΔV1 is applied inthe pixel A having the high mobility μ, then the drain-source currentIds drops by a great amount from Ids1′ to Ids1. On the other hand, sincethe feedback amount ΔV2 in the pixel B having the low mobility μ issmall, the drain-source current Ids decreases from Ids2′ to Ids2 anddoes not drop by a great amount. As a result, the drain-source currentIds1 in the pixel A and the drain-source current Ids2 in the pixel Bbecome substantially equal to each other, and consequently, thedispersion of the mobility μ among the pixels is corrected.

In summary, where the pixel A and the pixel B which are different in themobility μ therebetween are considered, the feedback amount ΔV1 in thepixel A having the high mobility μ is greater than the feedback amountΔV2 in the pixel B having the low mobility μ. In short, as the mobilityμ increases, the feedback amount ΔV increases and the reduction amountof the drain-source current Ids increases.

Accordingly, if the negative feedback is applied to the gate-sourcevoltage Vgs with the feedback amount ΔV in accordance with thedrain-source current Ids of the driving transistor 22, then the currentvalue of the drain-source current Ids is uniformized among the pixelswhich are different in the mobility μ from each other. As a result, thedispersion of the mobility μ among the pixels can be corrected. Thus,the process of applying negative feedback to the gate-source voltage Vgsof the driving transistor 22 with the feedback amount ΔV in accordancewith the current flowing through the driving transistor 22, that is,with the drain-source current Ids, is the mobility correction process.

Here, a relationship between the signal voltage or sampling potentialVsig of the image signal and the drain-source current Ids of the drivingtransistor 22 depending upon whether or not threshold value correctionand mobility correction are carried out in the pixel or pixel circuit 20shown in FIG. 2 is described with reference to FIGS. 9A to 9C.

FIG. 9A illustrates the relationship in a case wherein none of thethreshold value correction process and the mobility correction processis carried out, and FIG. 9B illustrates the relationship in another casewherein only the threshold value correction process is carried outwithout carrying out the mobility correction process while FIG. 9Cillustrates the relationship in a further case wherein both of thethreshold value correction process and the mobility correction processare carried out. As seen in FIG. 9A, when none of the threshold valuecorrection process and the mobility correction process is carried out,the drain-source current Ids is much different between the pixels A andB arising from a dispersion of the threshold potential Vth and themobility μ between the pixels A and B.

In contrast, where only the threshold value correction process iscarried out, although the dispersion of the drain-source current Ids canbe reduced to some degree as seen in FIG. 9B, the difference in thedrain-source current Ids between the pixels A and B arising from thedispersion of the mobility μ between the pixels A and B remains. Then,if both of the threshold value correction process and the mobilitycorrection process are carried out, then the difference in thedrain-source current Ids between the pixels A and B arising from thedispersion of the mobility μ for each of the pixels A and B can bealmost eliminated as seen in FIG. 9C. Accordingly, at any gradation, aluminance dispersion among the organic EL elements 21 does not appear,and a display image of favorable picture quality can be obtained.

Further, since the pixel 20 shown in FIG. 2 has a function of abootstrap operation by the storage capacitor 24 described hereinabove inaddition to the correction functions for threshold value correction andmobility correction, the following operation and effects can beachieved.

In particular, even if the source potential Vs of the driving transistor22 varies together with aged deterioration of the I-V characteristic ofthe organic EL element 21, the gate-source voltage Vgs of the drivingtransistor 22 can be kept fixed by a bootstrap operation by the storagecapacitor 24. Accordingly, the current flowing through the organic ELelement 21 does not vary but is fixed. As a result, since also theemitted light luminance of the organic EL element 21 is kept fixed, evenif the I-V characteristic of the organic EL element 21 undergoes ageddeterioration, image display which is free from luminance deteriorationby the aged deterioration can be implemented.

(Fault by a Shift of the Threshold Voltage of the Writing Transistor)

Here, the operating point of the writing transistor 23 when the organicEL element 21 emits light, particularly when the organic EL element 21displays the white, is studied. As apparent from the circuit operationdescribed above, after writing of the signal voltage Vsig of the imagesignal ends and the writing transistor 23 enters a non-conducting state,the gate voltage Vg of the driving transistor 22 rises in aninterlocking relationship with a rise of the source voltage Vs through abootstrap operation. Therefore, the gate voltage Vg of the drivingtransistor 22 becomes higher than the signal voltage Vsig.

On the other hand, if a configuration for applying the referencepotential Vofs for the initialization of the gate voltage Vg of thedriving transistor 22 is applied through the signal line 33 in order toexecute the threshold value correction process is adopted, then thepotential of the signal line 33 exhibits repetitions of changeoverbetween the reference voltage Vofs and the signal voltage Vsig in aperiod of 1H.

FIG. 10 illustrates a potential relationship of the electrodes of thewriting transistor 23 upon white display. Upon white display, an offvoltage Vssws for placing the writing transistor 23 into anon-conducting state is applied to the gate electrode G of the writingtransistor 23 and the reference voltage Vofs is applied to the sourceelectrode S of the writing transistor 23 while a white voltage Vwcorresponding to the white gradation is applied to the drain electrode Dof the writing transistor 23. The off voltage Vssws, reference voltageVofs and white voltage Vw have a voltage relationship of Vssws<Vofs<Vw.

FIG. 11 shows an example of a sectional structure of the writingtransistor 23. Referring to FIG. 11, a gate electrode 231 is formed frommolybdenum (Mo) or the like on a substrate which corresponds to theglass substrate 201 shown in FIG. 3, and a semiconductor layer 233 of,for example, amorphous silicon (a-Si) is layered on the gate electrode231 with a gate insulating film 232 interposed therebetween.

A portion of the semiconductor layer (a-Si) 233 which opposes to thegate electrode 231 forms a channel formation region. An insulating film234 is formed on the channel formation region. A source electrode 235and a drain electrode 236 both made of aluminum (Al) or the like areelectrically connected to a source region and a drain region of thesemiconductor layer 233, respectively, between which the channelformation region is sandwiched.

In the writing transistor 23 having the configuration described above,when the white is to be displayed, the off voltage Vssws is applied tothe gate electrode 231 and the white voltage Vw is applied to the drainelectrode 236 so that a high electric field is formed between the gateelectrode 231 and the drain electrode 236. Here, the drain voltage ofthe writing transistor 23 is equal to the gate voltage of the drivingtransistor.

If an electric field continues to be generated between the gateelectrode 231 and the drain electrode 236 of the writing transistor 23,then electrons in the semiconductor layer 233 which are to form thechannel are trapped in the insulating film 234 positioned above thesemiconductor layer 233 and tend to generate a reverse electric field ina direction in which the electric field is to be canceled. Since thetrapped electrons exist also when the writing transistor 23 conducts,the threshold voltage Vthws of the writing transistor 23 is shifted orfluctuated to the negative side by the reverse electric field. Thephenomenon that the threshold voltage Vthws is shifted to the negativeside appears conspicuously as time passes.

Incidentally, as the increase in size and definition of a display paneladvances, the wiring line resistance and the parasitic capacitance ofthe scanning line 31 for transmitting the writing scanning signal WS inthe form of a pulse to be applied to the gate electrode of the writingtransistor 23 increase. Then, where the wiring line resistance or theparasitic capacitance of the scanning line 31 increases, as the distancefrom the input end of the display panel 70 increases, the waveform ofthe writing scanning signal WS becomes blunt.

Meanwhile, the mobility correction process is executed in parallel tothe writing process of the signal voltage Vsig of the image signal bythe writing transistor 23. As apparent from the timing waveform diagramof FIG. 4, the mobility correction period, that is, the signal writingperiod, depends upon the waveform of the writing scanning signal WS.Therefore, if the threshold voltage Vthws of the writing transistor 23shifts to the negative side in a state wherein the waveform of thewriting scanning signal WS is blunt, then the mobility correction timewhen the white is to be displayed or the black is to be displayedbecomes longer by a period of time corresponding to the shift amount ofthe threshold voltage.

FIG. 12 illustrates a transition waveform of the writing scanning signalWS in a state wherein it is blunt at a rising edge and a fall edgethereof. Referring to FIG. 12, reference character VsigW denotes a whitesignal voltage corresponding to the white gradation; VsigB a blacksignal voltage corresponding to the black gradation; and ΔVthws a shiftamount of the threshold voltage Vthws of the writing transistor 23.

As can be seen apparently from the waveform diagram of FIG. 12,particularly when the white or the black is displayed, if the thresholdvoltage Vthws of the writing transistor 23 shifts by the shift amountΔVthws to the negative side, then the mobility correction periodincreases by the shift amount ΔVthws of the threshold voltage Vthws.This variation of the threshold voltage Vthws appears conspicuouslyparticularly at a falling edge of the waveform of the writing scanningsignal WS. The reason is such as described below.

As seen from the waveform diagram of FIG. 12, in the transition waveformof the writing scanning signal WS, a transition ending portion of therising/falling edges exhibits a higher degree of blunting of thewaveform than a transition starting portion of the rising/falling edges.The amplitude of the white signal voltage VsigW is equal to or smallerthan one half that of the writing scanning signal WS. Accordingly, asapparently seen from the waveform diagram of FIG. 12, the variation ofthe mobility correction period arising from the shift of the thresholdvoltage Vthws of the writing transistor 23 to the negative side appearsconspicuously particularly at the falling edge of the waveform of thewriting scanning signal WS.

Further, as apparent from the foregoing description of the circuitoperation, the mobility correction process is carried out while thesource voltage Vs of the driving transistor 22 is raised. Therefore, asthe mobility correction period increases, the rises of the sourcevoltage Vs of the driving transistor 22 increases. Consequently, thegate-source voltage Vgs of the driving transistor 22 drops and thecurrent to flow to the organic EL element 21 decreases, and as a result,the emitted light luminance decreases as time passes or such a picturequality defect as stripes or luminance unevenness appears.

Further, as described hereinabove, also when light of high energy isinputted to the channel of the writing transistor 23, the thresholdvoltage of the writing transistor 23 shifts to the negative side (referto FIG. 26). Where the R, G and B pixels (subpixels) are disposed suchthat the B pixel is positioned centrally, the B pixel is influenced byblue light only when it itself emits light.

On the other hand, since the R and G pixels are positioned adjacent theB pixel, they are influenced by emitted light of the B pixel even whenthey themselves emit no light. At this time, not only the writingtransistor 23 but also the driving transistor 22 is influenced by bluelight such that the characteristic thereof is shifted. Where not only aninfluence of self light emission but also an influence of light emissionof adjacent pixels is had, it is very difficult to compensate for thecurrent variation in correction processes such as the mobilitycorrection process.

As a method of preventing an influence of blue light from an adjacentpixel, a method seems applicable wherein the writing transistor 23 iscovered with a metal wiring layer 301 in the same layer as that of theanode electrode of the organic EL element 21 as seen in FIG. 13 to blockblue light. However, with the method described, although a lightblocking effect can be expected to some degree, since it is necessary toform a flattening film 302, which corresponds to the insulatingflattening film 203 shown in FIG. 3, with an increased thickness inorder to maintain the flatness, full light blocking cannot be expectedeven if the metal wiring layer 301 is disposed on the writing transistor23.

Characteristics of the Present Embodiment

Therefore, the present embodiment adopts the following pixel lightblocking layout in an organic EL display apparatus wherein a pluralityof subpixels which form one pixel which forms a unit in formation of acolor image are disposed adjacent each other. In particular, the presentembodiment adopts a light blocking layout structure wherein, at leastfor a second subpixel from among the plural subpixels which ispositioned adjacent a first subpixel which emits light of the shortestwavelength, a light blocking member having a width greater than thechannel length or the channel width of a transistor which forms thesecond subpixel is provided so as to be positioned between the first andsecond subpixels.

Although description here is given of a case wherein the pluralsubpixels which form one pixel which forms a unit in formation of acolor image are formed, for example, from a combination of R, G and Bpixels or subpixels, they are not limited to the specific combination.In the case of the combination of R, G and B pixels, the emitted lightfrom the B pixel has the shortest wavelength. Accordingly, the B pixelserves as the first subpixel, and each of the R and G pixels serves asthe second subpixel.

The transistor which forms the second subpixel may be, for example, thewriting transistor 23. However, as described above, not only acharacteristic shift of the writing transistor 23 is caused by aninfluence of blue light of the B pixel, but also a characteristic shiftof the driving transistor 22 is caused by an influence of blue light.Accordingly, the transistor which serves as the second subpixel is notlimited to the writing transistor 23.

Where a light blocking member is provided for the second subpixel withrespect to the first subpixel such that the width thereof is greaterthan the channel length or the channel width of the transistor whichforms the second subpixel as described above, light emitted from thefirst subpixel can be blocked with certainty. Accordingly, acharacteristic shift caused by inputting of light having high energy tothe channel of the transistor which forms the second subpixel,particularly a shift of the threshold voltage Vth to the negative side,can be suppressed.

Since a shift of the threshold voltage of the writing transistor 23 issuppressed, the fluctuation of the mobility correction period or signalwriting period which depends upon the waveform of the writing scanningsignal WS can be reduced. Where the fluctuation of the mobilitycorrection period is reduced, the rise of the source voltage Vs of thedriving transistor 22 arising from the fluctuation can be suppressed.Therefore, reduction of the current to flow to the organic EL element 21is suppressed, and consequently, reduction of the emitted lightluminance with respect to time and occurrence of a fault in picturequality such as stripes or luminance unevenness can be suppressed.

Such effects as described above can be obtained from the light blockinglayout structure for a pixel according to the present embodiment. In thefollowing, particular working examples of the light blocking layoutstructure are described.

1-1. Working Example 1

FIG. 14 is a plan view showing the light blocking layout structureaccording to a working example 1. Here, as an example, the lightblocking layout structure exhibits a color array wherein R, G and Bpixels or subpixels 20R, 20G and 20B are arranged such that the B pixel20B is positioned centrally and the R and G pixels 20R and 20B arepositioned on the opposite sides of the B pixel 20B. FIG. 15 shows asectional structure of the light blocking layout structure taken alongline A-A′ of FIG. 14.

Since the B pixel 20B is disposed centrally as seen in FIG. 14,transistors in the R and G pixels 20R and 20G positioned on the oppositesides of the B pixel 20B are influenced by irradiation of blue lightemitted from the B pixel 20B. In order to prevent incidence of bluelight from the B pixel 20B, a metal wiring layer 301G is provided in thelayer same as that of the anode electrode of the G pixel 20G on thewriting transistor 23, on the organic EL element 21 side of FIG. 3, withthe flattening film 302 interposed therebetween similarly as in the caseof the light blocking layout structure described hereinabove withreference to FIG. 13. The metal wiring layer 301G is formed from a metalmaterial having high reflectivity such as aluminum and same as that ofthe wiring line for the anode electrode.

The G pixel 20G further includes a light blocking member 303G providedin parallel to the longitudinal direction of the G pixel 20G between theG pixel 20G and the B pixel 20B in a region of the metal wiring layer301G as shown in FIGS. 14 and 15. The light blocking member 303G has awidth greater than the channel width of the writing transistor 23 asapparently seen particularly from FIG. 14 and is embedded in a holeformed in the metal wiring layer 301G. As the material of the lightblocking member 303G, for example, a material same as that of the metalwiring layer 301G, that is, a metal material having high reflectivitysuch as aluminum, is used.

While the light blocking layout structure of the G pixel 20G isdescribed here, also the R pixel 20R has a light blocking layoutstructure basically same as that of the G pixel 20G. The light blockinglayout structure of the G pixel 20G and the light blocking layoutstructure of the R pixel 20R are symmetrical to each other with respectto the center line of the B pixel 20B.

Where the light blocking members 303G and 303R are provided between theB pixel 20B and the R and G pixels 20R and 20B positioned adjacent toand on the opposite sides of the B pixel 20B in this manner,respectively, blue light emitted from the B pixel 20B can be blockedwith certainty so as not to be inputted to the pixels 20R and 20G.Therefore, the characteristic shift by an influence of irradiation ofblue light upon the channel of the writing transistor 23 can besuppressed low, and consequently, reduction of the current to flow tothe organic EL element 21 and occurrence of a fault in picture qualitysuch as stripes or luminance unevenness can be suppressed.

Modification 1 to the Working Example 1

FIG. 16 is a plan view showing a light blocking layout structureaccording to a modification 1 to the working example 1. In the lightblocking layout structure according to the modification 1, lightblocking members 303B-1 and 303B-2 having a width greater than thechannel width of the writing transistor 23 are provided between R and Gpixels 20R and 20B positioned adjacent to and on the opposite sides ofthe B pixel 20B.

With the light blocking layout structure according to the modification1, blue light emitted from the B pixel 20B can be prevented from beingreflected by the light blocking members 303G and 303R of the adjacentpixels and entering the B pixel 20B. Accordingly, not only with regardto the R and G pixels 20R and 20B but also with regard to the B pixel20B, the characteristic shift by an influence of irradiation of bluelight can be suppressed to a small amount.

Modification 2 to the Working Example 1

FIG. 17 is a plan view showing a light blocking layout structureaccording to a modification 2 to the working example 1. Referring toFIG. 17, in the light blocking layout structure according to presentmodification 2, each of the pixels 20G, 20B and 20R is blocked againstlight by light blocking members 303-1 to 303-4 in the four directions ofleftward, rightward, upward and downward directions of the writingtransistor 23.

In particular, the writing transistor 23 in the G pixel 20G is blockedon the left and right thereof against light by the light blockingmembers 303G-1 and 303G-2. The light blocking members 303G-1 and 303G-2have a width greater than the channel width of the writing transistor23. Further, the writing transistor 23 is blocked on the upper and lowersides thereof against light by the light blocking members 303G-3 and303G-4. The light blocking members 303G-3 and 303G-4 have a widthgreater than the channel length of the writing transistor 23.

Also for the B and R pixels 20B and 20R, a light blocking layoutstructure basically similar to that for the G pixel 20G is provided.

With the light blocking layout structure according to the presentmodification 2, the writing transistors 23 in the pixels 20G, 20B and20R can be shielded optically substantially fully from blue lightemitted from the B pixel 20B. Accordingly, with regard to each of thepixels 20G, 20B and 20R, the characteristic shift by an influence ofirradiation of blue light can be suppressed with a higher degree ofcertainty.

Modification 3 to the Working Example 1

FIG. 18 is a sectional view showing a light blocking layout structureaccording to a modification 3 to the working example 1. Referring toFIG. 18, in the light blocking layout structure according to the thirdmodification, a lower end portion of a light blocking member 303B isembedded in a gate insulating film 232 which corresponds to theinsulating film 202 shown in FIG. 3.

While the light blocking member 303B for the B pixel 20B is shown inFIG. 18, also the light blocking members 303B-1 and 303B-2 in themodification 1 and the insulating films 303G-1 to 303G-4 in themodification 2 can be embedded at a lower end portion thereof in a gateinsulating film 304. The foregoing description applies similarly to theB and R pixels 20B ad 20R.

With the light blocking layout structure according to the presentmodification 3, since the lower end portion of the light blocking member303B is embedded in the gate insulating film 304, also leakage lightfrom between the lower end portion of the light blocking member 303B andthe gate insulating film 304 can be blocked with certainty. Accordingly,in each of the pixels 20G, 20B and 20R, the characteristic shift by aninfluence of irradiation of blue light can be suppressed with certainty.

Modification 4 to the Working Example 1

FIG. 19 is a sectional view showing a light blocking layout structureaccording to a modification 4 to the working example 1. Referring toFIG. 19, in the light blocking layout structure according to themodification 4 to the working example 1, a lower end portion of thelight blocking member 303B is electrically connected to a wiring line305 for the source electrode of the driving transistor 22.

With the light blocking layout structure according to the presentmodification 4, since the light blocking member 303B establisheselectric connection between the source electrode of the drivingtransistor 22 and the anode electrode of the organic EL element 21, thelight blocking member 303B can be used also as a contact portion for theelectric connection. Consequently, the light blocking layout structureagainst blue light can be implemented without inviting complication ofthe pixel structure by the provision of the light blocking member 303B.

1-2. Working Example 2

FIG. 20 is a sectional view showing a light blocking layout structureaccording to a working example 2.

In the light blocking layout structure according to the working example1, the light blocking member 303G is provided below the metal wiringlayer 301G. In contrast, in the light blocking layout structureaccording to the present working example 2, the light blocking member303G is provided below an auxiliary line 306 provided in the same layeras that of the metal wiring layer 301G between the pixels. Here, theauxiliary line 306 is normally disposed so as to surround each of thepixels in order to supply the cathode potential Vcath to the cathodeelectrode of the organic EL element 21.

For the light blocking member 303G, for example, a material same as thatof the auxiliary line 306, that is, a metal material having highreflectivity such as aluminum, is used. While the light blocking member303B for the B pixel is shown in FIG. 20, also the insulating films303G-1 to 303G-4 in the modification 1 or the modification 2 to theworking example 1 may be embedded at a lower end portion thereof in thegate insulating film 304. This similarly applies also to the B and Rpixels 20B ad 20R.

In this manner, also where the light blocking member 303B is providedbelow the auxiliary line 306, it is possible to block blue light emittedfrom the B pixel 20B from being inputted to the R and G pixels 20R and20B similarly as in the working example 1. In addition, the locationbelow the auxiliary line 306 is outside the pixels, and there is anadvantage that it is easier to produce the light blocking member 303G inregard to the space than where the auxiliary line 306 is provided belowthe metal wiring layer 301G and a large aperture can be formed.

2. Modifications

In the embodiment described above, the driving circuit for the organicEL element 21 basically has a 2-Tr configuration which includes twotransistors (Tr) including the driving transistor 22 and the writingtransistor 23, the present invention is not limited to the 2-Trconfiguration. In particular, the driving circuit can have various pixelconfigurations such as, a pixel configuration which includes atransistor for controlling light emission/no-light emission of theorganic EL element 21 in addition to the two transistors and anotherpixel configuration which additionally includes a switching transistorfor selectively writing the reference voltage Vofs into the gateelectrode of the driving transistor 22.

Further, while, in the embodiment described above, the present inventionis applied to an organic EL display apparatus which uses an organic ELelement as an electro-optical element of a pixel, the present inventionis not limited to this application. In particular, the present inventioncan be applied to various display apparatus which use an electro-opticalelement, that is, a light emitting element, of the current driven typewhose emitted light luminance varies in response to the value of currentflowing to the element such as an inorganic EL element, an LED elementor a semiconductor laser element.

3. Applications

The display apparatus according to an embodiment of the presentinvention described above can be applied to a display apparatus forelectronic apparatus in various fields wherein an image signal inputtedto the electronic apparatus or an image signal produced in theelectronic apparatus is displayed as an image.

With the display apparatus according to an embodiment of the presentinvention, it is possible to suppress a characteristic shift caused byan influence of irradiation of light having high energy upon the channelof a pixel transistor thereby to suppress reduction of current to flowto the organic EL element and appearance of a fault in picture qualitysuch as stripes and luminance unevenness. Accordingly, by using thedisplay apparatus according to an embodiment of the present invention asa display apparatus of electronic apparatus in various fields,enhancement of the display quality of the display apparatus of theelectronic apparatus can be anticipated.

The display apparatus according an embodiment of the present inventionmay be of the module type having an enclosed configuration. The displayapparatus of the module type corresponds, for example, to a displaymodule wherein an opposing element of transparent glass or the like isadhered to a display array section. On the transparent opposing portion,a color filter, a protective film and so forth as well as the lightblocking film described hereinabove may be provided. It is to be notedthat the display module may include a circuit section for inputting andoutputting signals and so forth from the outside to the pixel arraysection and vice versa, a flexible printed circuit board (FPC) and soforth.

In the following, particular examples of an electronic apparatus towhich an embodiment of the present invention is applied are described.In particular, the present invention can be applied to such variouselectronic apparatus as shown in FIGS. 21 to 25A to 25G, for example, toa digital camera, a notebook type personal computer, a portable terminalapparatus such as a portable telephone set and a video camera.

FIG. 21 shows an appearance of a television set to which an embodimentof the present invention is applied. Referring to FIG. 21, thetelevision set shown includes a front panel 102 and an image displayscreen section 101 formed from a filter glass plate 103 and so forth andis produced using the display apparatus according to the embodiment ofthe present invention as the image display screen section 101.

FIGS. 22A and 22B show an appearance of a digital camera to which anembodiment of the present invention is applied. Referring to FIGS. 22Aand 22B, the digital camera shown includes a flash light emittingsection 111, a display section 112, a menu switch 113, a shutter button114 and so forth. The digital camera is produced using the displayapparatus according to the embodiment of the present invention as thedisplay section 112.

FIG. 23 shows an appearance of a notebook type personal computer towhich an embodiment of the present invention is applied. Referring toFIG. 23, the notebook type personal computer shown includes a body 121,and a keyboard 122 for being operated in order to input characters andso forth, a display section 123 for displaying an image and so forthprovided on the body 121. The notebook type personal computer isproduced using the display apparatus according to the embodiment of thepresent invention as the display section 123.

FIG. 24 shows an appearance of a video camera to which an embodiment ofthe present invention is applied. Referring to FIG. 24, the video camerashown includes a body section 131, and a lens 132 for picking up animage of an image pickup object, a start/stop switch 133 for imagepickup, a display section 134 and so forth provided on a face of thebody section 131 which is directed forwardly. The video camera isproduced using the display apparatus according to the embodiment of thepresent invention as the display section 134.

FIGS. 25A to 25G show an appearance of a portable terminal apparatus,for example, a portable telephone set, to which an embodiment of thepresent invention is applied. Referring to FIGS. 25A to 25G, theportable telephone set includes an upper side housing 141, a lower sidehousing 142, a connection section 143 in the form of a hinge section, adisplay section 144, a sub display section 145, a picture light 146, acamera 147 and so forth. The portable telephone set is produced usingthe display apparatus of the embodiment of the present invention as thedisplay section 144 or the sub display section 145.

The present application contains subject matter related to thatdisclosed in Japanese Priority Patent Application JP 2008-325072 filedin the Japan Patent Office on Dec. 22, 2008, the entire content of whichis hereby incorporated by reference.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factor in so far as they arewithin the scope of the appended claims or the equivalents thereof.

1. A display apparatus, comprising: a plurality of subpixels disposedadjacent each other and forming one pixel which forms a unit forformation of a color image; said plurality of subpixels including afirst subpixel which emits light of the shortest wavelength and a secondsubpixel disposed adjacent said first subpixel; said second subpixelhaving a light blocking member disposed between said second subpixel andsaid first subpixel and having a width greater than a channel length ora channel width of a transistor which forms said second subpixel.
 2. Thedisplay apparatus according to claim 1, wherein said light blockingmember is disposed in parallel to the longitudinal direction of saidsecond subpixel.
 3. The display apparatus according to claim 1, whereinsaid light blocking member is disposed in such a manner as to opticallycover the transistor of said second subpixel against the emitted lightof said first subpixel.
 4. The display apparatus according to claim 1,wherein said light blocking member is made of a material same as that ofa metal line which forms an anode electrode of an electro-opticalelement in said second subpixel.
 5. The display apparatus according toclaim 1, wherein said light blocking member is disposed below anauxiliary line wired between said first subpixel and said secondsubpixel.
 6. The display apparatus according to claim 5, wherein saidlight blocking member is made of a material same as that of saidauxiliary line.
 7. The display apparatus according to claim 1, whereinsaid first subpixel has a light blocking member disposed between saidfirst subpixel and said second subpixel and having a width greater thana channel length or a channel width of a transistor which forms saidfirst subpixel.
 8. The display apparatus according to claim 1, whereineach of said plurality of subpixels includes a writing transistor forwriting an image signal and a driving transistor for driving anelectro-optical element in response to the image signal written by saidwriting transistor.
 9. The display apparatus according to claim 8,wherein each of said plurality of subpixels has a function for amobility correction process of correcting a mobility of said drivingtransistor by negatively feeding back a correction amount correspondingto current flowing to said driving transistor to a potential differencebetween a gate and a source of said driving transistor.
 10. The displayapparatus according to claim 9, wherein the mobility correction processis carried out in parallel to the writing process of the image signal bysaid writing transistor.
 11. The display apparatus according to claim 9,wherein the mobility correction process is carried out while a sourcevoltage of said driving transistor is raised.
 12. An electronicapparatus, comprising: a display apparatus including a plurality ofsubpixels disposed adjacent each other and forming one pixel which formsa unit for formation of a color image, said plurality of subpixelsincluding a first subpixel which emits light of the shortest wavelengthand a second subpixel disposed adjacent said first subpixel, said secondsubpixel having a light blocking member disposed between said secondsubpixel and said first subpixel and having a width greater than achannel length or a channel width of a transistor which forms saidsecond subpixel.